Buying paint by guesswork leads to either a half-finished wall and a second shop trip, or a cupboard full of leftover tins. A quick calculation based on wall area, coats and coverage gets it right.
How the calculation works
- Wall area = perimeter of the room × wall height.
- Subtract openings: roughly 21 sq ft per door and 15 sq ft per window.
- Add the ceiling if you are painting it.
- Multiply by coats, usually two for a good finish.
- Divide by coverage, the area one litre covers per coat. This is printed on the tin and depends on the paint and the surface.
Coverage varies more than you think
Smooth, primed, previously painted walls need far less paint than new rough plaster. Dark colours over light walls, or light over dark, often need an extra coat. Always check the coverage figure on the specific product you are buying.
Paint types for homes
| Type | Where | Notes |
|---|---|---|
| Distemper | Interior, budget | Cheapest, not washable |
| Interior emulsion | Living rooms, bedrooms | Washable, many finishes from matt to sheen |
| Exterior emulsion | Outside walls | Weather and algae resistant; premium grades have elastomeric properties |
| Enamel | Doors, grills, metal | Hard, glossy finish |
The steps that matter more than the paint
- Dry walls: new plaster must dry thoroughly. Painting damp walls causes peeling.
- Primer: use an alkali-resistant primer on new cement plaster.
- Putty: wall putty fills small dents for a smooth finish on interior walls.
- Fix damp first: paint will not stop seepage. See our damp walls guide.
Frequently asked questions
How long between coats?
Usually a few hours for water-based emulsions, but follow the time on the tin, especially in humid weather.
Should I buy a little extra?
Yes. Buy about 10% extra and keep a small amount from the same batch for future touch-ups, because colours can vary slightly between batches.